1. Field of the Invention
This invention relates generally to the field of mechanics, aerodynamics, and propulsion systems. More specifically, the invention describes a solid propellant air-turborocket.
2. Description of the Prior Art
An air-turborocket (ATR), also known as air-turboramjet, fan-boosted ramjet, and gas generator turbojet motor, is an airbreathing jet propulsion engine in which ram air compression is supplemented by a mechanical compressor driven by a turbine. The turbine is driven by the exhaust of a gas generator using a fuel-rich liquid or solid propellant. The tubine exhaust gases constitute the fuel, which is burned in the compressed air behind the turbocompressor assembly. Judicious choice of the propellant, compressor pressure ratio, and component efficiencies establishes burner stoichiometry and performance characteristics of the engine.
Past and present ATRs operate well both subsonically and supersonically and do not need supplemental propulsion such as a rocket booster or high speed launch to boost the ATR to a satisfactory operating speed as a ramjet does. The high thrust/weight ratios of ATRs permit ATR propelled missiles to postpone acceleration to supersonic speeds until such speeds are needed for survival or maneuverability close to a target. Thus far, most ATR hardware development has used only liquid propellants; however, solid propellant ATRs offer performance very similar to or superior to engines using high performance liquid monopropellants such as a mixed hydrazine fuel.
Existing solid propellant rockets have been used as boosters for ramjet sustainers. A problem associated therewith is the relatively low specific impulse of the solid propellant in comparison to liquid propellants. The weight of solid propellant required to obtain a given impulse for propelling a missile is appreciably greater than the weight of liquid propellant that would produce the same impulse. If the specific impulse of the solid propellants were greater, missiles using them could have a lower initial weight and perform the same function with reduced hazards. This would be an advantage in the production, cost, and handling of the missiles.
Solid propellants consist primarily of granular oxidizer dispersed in a binder fuel. The weight ratio of oxidizer to binder varies from one composition to another, but it generally is in the ratio of 75 to 85 percent oxidizer with the remaining percentage being fuel. This high ratio of oxidizer is required to insure complete combustion of the fuel. If some of the fuel can be combusted with oxygen from an outside source rather than from oxidizer contained in the propellant, less granular oxidizer would be needed in the propellant. Thrust obtained from the complete combustion of the fuel remains approximately the same regardless of the source of the oxidizer.
An alternative to combusting solid propellant fuel entirely from granular oxidizer dispersed in the propellant is to use oxygen from surrounding air as a partial source of oxidizer. This is achieved in a ducted rocket ramjet by placing a shroud around the nozzle of a fuel rich solid propellant motor. Air is admitted at the forward end of the shroud and mixes with fuel-rich combustion products from the motor to produce additional burning, gas expansion, and thrust when discharged from the aft end of the shroud. The heretofore proposed ducted rocket ramjets employ booster charges or motors. The booster, which is a conventional solid propellant rocket, overcomes the added drag and weight of the shroud, accelerates the missile to ramjet velocities, and the booster, or certain parts of it are then separated.
The liquid or solid propellant ATRs, similar in concept to this invention, utilize a turbine-driven air compressor in conjunction with afterburners to improve specific impulse. The well known trade offs between liquid and solid propulsion systems are applicable here. Solid propellants offer substantial advantages in lower manufacturing cost, safety, simplicity, storability, logistics, and reliability. The use of compressed atmospheric air to augment the thrust of solid propellant rockets is especially attractive for relatively large diameter, long range missiles delivering heavy payloads.
Existing solid propellant ATRs, however, have been designed with less efficiency than is desirable. Hot expanding gases from the solid propellant which drive the turbine for the air compressor in existing solid propellant ATRs provide a poor match between turbine and compressor blade speeds. In addition, the rotor disk and accompanying bearing surfaces from which the compressor blades radiate are exposed to the hot expanding fuel rich gases of existing solid propellant ATRs, a very undesirable situation. Further, in addition to the above, there remains a continuing need for forcibly mixing the hot expanding fuel rich gases with the compressed airstream in the thrust combustion chamber to minimize the length and weight of the chamber. In consideration of the foregoing prior art limitations to present day needs, the invention disclosed herein was conceived.